Feed forward control of expansion valve

ABSTRACT

A method of controlling an expansion valve in a refrigeration system. The method comprises the steps of controlling the modulation of the expansion valve based upon a first feedback control criteria; and controlling the modulation of the expansion valve based upon a second feed forward control criteria.

BACKGROUND OF THE INVENTION

The present invention is directed to a feed forward control for theexpansion valve of a heating, ventilating, air conditioning orrefrigeration (HVAC/R) system. In the preferred embodiment of theinvention as described herein, the HVAC/R system is a water chillersystem. Although discussed in terms of water chillers, the invention isapplicable to all HVAC and refrigeration systems having systemdisturbances which can be anticipated. Such system disturbances includecompressor staging, changes in compressor capacity such as those causedby loading or unloading, physical changes regarding the various coolingmedia used by the system such as changes in evaporator or condenserwater temperature, changes in condenser cooling capacity such as thosecaused by fan staging, changes in evaporator heat exchanger capacity,changes in setpoint, changes in cooling tower capacity, and changesresulting from building load variations.

In water chiller systems, water is chilled in an evaporator so as toprovide a cooling medium for air conditioning use elsewhere. Water ischeap, safe and can easily be transported by piping to an air handler bya first water loop. The air handler exchanges heat between air and waterso as to condition the air for use in a zone or building.

The evaporator in a water chiller system is controlling the temperatureof the water by heat exchange with refrigerant. The refrigerantcirculates throughout the chiller system by means of a refrigerant loop.In the refrigerant loop, the refrigerant leaves the evaporator andenters a compressor where the pressure of the refrigerant is increasedso as to change its condensation point. The compressed refrigerantleaves the compressor and enters a condenser where it is condensed froma vapor to a liquid refrigerant by heat exchange with a cooling medium,typically a second water system. The liquid refrigerant is thenreturned, by means of an expansion device, to the evaporator to continuethe cycle through the chiller system. Aspects of typical chiller systemsare shown in U.S. Pat. No. 4,780,061 to Butterworth; U.S. Pat. No.4,762,409 to Tischer; U.S. Pat. No. 4,730,995 to Dewhirst; U.S. Pat. No.4,662,190 to Tischer and U.S. Pat. No. 5,201,648 to Lakowske. All ofthese patents are assigned to the assignee of the present invention andall of these patents are incorporated herein by reference.

The expansion device in the chiller system is an electronic expansionvalve which modulates refrigerant flow through the expansion valve inresponse to refrigerant superheat as measured after the refrigerantleaves the compressor. Typical electronic expansion valves are shown inU.S. Pat. No. 5,083,745 to Tischer; U.S. Pat. No. 4,986,085 to Tischer;U.S. Pat. No. 4,928,494 to Glamm and U.S. Pat. No. 5,011,112 to Glamm.These patents are assigned to the assignee of the present invention andare hereby incorporated by reference.

Typically, the compressor capacity is modulated in response to theleaving water temperature of the evaporator. Various methods ofcompressor capacity control and chiller capacity control are shown inU.S. Pat. No. 5,027,608 to Rentmeester et al.; U.S. Pat. No. 5,203,685to Anderson et al.; U.S. Pat. No. 5,211,026 to Linnert; U.S. Pat. No.4,715,190 to Han et al. and U.S. Pat. No. 4,689,967 to Han et al. Eachof these patents is assigned to the assignee of the present inventionand is hereby incorporated by reference.

While these various methods of controlling the expansion device andcompressor capacities provide efficient and economical controls, bettercontrols are both possible and desirable. More specifically,conventional control of the expansion valve is accomplished by feedingback a signal representing the result of the expansion valve's actions,that result typically being measured superheat. Such a control strategy,while effective, is reactionary as opposed to anticipatory. Thus, theexpansion valve is constantly reacting to system disturbances such aschanges in compressor capacity.

SUMMARY OF THE INVENTION

It is the principle object of the present invention to provide bettercontrols for HVAC and refrigeration systems such as water chillersystems.

It is an object, feature and advantage of the present invention to linkthe control of an expansion valve directly to system disturbances suchas, for example, changes in load, changes in compressor capacity, orchanges in the temperatures of the various heat exchange fluids used inthe system.

It is an object, feature and advantage of the present invention toprovide a feed forward control which, during normal capacity control,allows the expansion valve to be positioned in response to compressorcapacity changes so as to anticipate the system disturbances.

It is an object, feature and advantage of the present invention toprovide an expansion valve which responds directly to changes incompressor capacity.

It is an object of the present invention to provide an expansion valvewhich responds directly to changes in the temperature of the waterentering the evaporator.

It is an object of the present invention to provide an expansion valvewhich responds directly to changes in the differential between thetemperature of the water entering the evaporator and the temperature ofthe water leaving the evaporator.

It is an object, feature and advantage of the present invention toprovide closed loop control of an expansion valve based upon a firstcriteria and open loop control of the expansion valve based upon asecond criteria.

It is a further object of the present invention that the first criteriabe discharge superheat and that the second criteria be either evaporatorentering water temperature or the difference between evaporator enteringwater temperature and evaporator leaving water temperature.

It is another object of the present invention that the first criteria bea measure of refrigerant liquid level in the evaporator and that thesecond criteria be a measure of compressor capacity.

It is an object, feature and advantage of the present invention to use afeed forward control signal from the compressor unloader to repositionthe expansion valve instantaneously as system disturbances occur therebyreducing swings in either discharge superheat or refrigerant liquidlevel.

It is an object, feature and advantage of the present invention todetermine and control the position of an electronic expansion valve as afunction of a signal or signals indicative of unloader position,saturated evaporator temperature, and saturated condensing temperature.

It is a further object of the present invention to reposition theexpansion valve instantaneously upon the occurrence of an unloaderposition change or a change in the saturated operating temperatures.

It is a further object of the present invention to control theevaporator liquid level by making adjustments in expansion valveposition if the liquid level drifts upwardly or downwardly.

It is a further object of the present invention to control dischargesuperheat by adjustments to the expansion valve position if thedischarge superheat drifts upwardly or downwardly.

It is a further object of the present invention to reposition theexpansion valve based on changes in motor current.

It is an object, feature and advantage of the present invention toprovide an expansion valve which will open to the calculated compressorpumping rate based upon the pressure differences across the expansionvalve and the refrigerant density.

The present invention provides a method of controlling an expansionvalve in a refrigeration system. The method comprises the steps ofcontrolling the modulation of the expansion valve based upon a firstfeedback control criteria; and controlling the modulation of theexpansion valve based upon a second feedforward control criteria.

The present invention also provides a method of controlling the flow ofrefrigerant in a refrigeration system including an expansion device anda compression device. The method comprises the steps of measuring thepumping capacity or CFM displacement capacity of the compression device;and modulating the expansion device to match the measured compressorcapacity.

The present invention further provides a method of controlling anexpansion valve comprising the steps of: (a) constantly controlling anexpansion valve based upon a first measured criteria; (b) determining adisturbance, in a second measured criteria, in the system beingcontrolled by the expansion valve; (c) providing a feed forward controlsignal to the expansion valve based upon the second criteria; and (d)controlling the expansion valve based upon the second criteria.

The present invention still further provides a method of controlling anexpansion valve in a refrigeration or HVAC system comprising the stepsof: providing closed loop control of an expansion valve based upon afirst criteria; and providing open loop control of the expansion valvebased upon at least a second criteria differing from the first criteria.

The present invention additionally provides a method of controlling thecapacity of a chiller system where the chiller system includes acircularly linked compressor, condenser, expansion device, and anevaporator. In the system, the evaporator receives entering water havingan entering water temperature and provides leaving water having aleaving water temperature and where the compressor compressesrefrigerant having a measured superheat. The method comprising the stepsof: measuring the leaving water temperature and the refrigerantsuperheat; determining a leaving water temperature error as a functionof the difference between the leaving water temperature and a setpoint;providing a first mode of capacity control wherein the expansion valveis modulated in response to the measured superheat; and providing asecond mode of capacity control where the expansion valve is modulatedin response to the leaving water temperature error or a change incompressor capacity.

The present invention yet further provides a method of repositioning anexpansion valve in a chiller system in response to changes in systemcapacity. The chiller system has an evaporator for conditioning enteringwater and providing leaving water. The method comprising the steps of:monitoring the entering water temperature of the evaporator; monitoringthe leaving water temperature of the evaporator; determining a watertemperature difference between the entering water temperature and theleaving water temperature; determining a temperature differentialsetpoint; comparing the water temperature difference to the differentialtemperature setpoint to establish an error; and repositioning theexpansion valve to minimize the error.

The present invention additionally provides a water chiller systemcomprising an evaporator for chilling a fluid; and a compressor,receiving heated refrigerant from the evaporator, for changing thecondensation temperature of the refrigerant by compression. The systemalso includes a condenser, receiving compressed refrigerant from thecompressor, for condensing the compressed refrigerant; an expansiondevice, receiving condensed refrigerant from a condenser, forcontrollably returning the condensed refrigerant to the evaporator; anda controller, operatively connected to the expansion device, forcontrolling the operation of the expansion device. The system furtherincludes a feed forward control, coupled to the controller, foranticipating changes in the load of the evaporator whereby the feedforward control includes a first sensor sensing a first value andcontrolling the operation of the expansion valve in response to thesensed first value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a water chiller system to which the feedforward concept of the present invention applies.

FIG. 2 is an alternative embodiment including a spray tree evaporatorwherein the expansion valve responds to changes in compressor capacitythereby minimizing disturbances in liquid levels.

FIG. 3 is a control block diagram of the operation of the presentinvention.

FIG. 4 is a table showing the first and second preferred embodiments andthe controls used during steady state conditions and system disturbanceconditions.

FIG. 5 is a graph of the first preferred embodiment.

FIG. 6 is a graph of the second preferred embodiment.

FIG. 7 illustrates the unloader arrangement for a screw compressor.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical chiller system 10 which uses refrigerant toprovide chilled water for air conditioning purposes. The chiller system10 has a refrigerant loop 12 including an evaporator 20, an expansiondevice such as an expansion valve 30, a condenser 40 and a compressor50. The entire system is controlled by an electronic controller 60.

The evaporator 20 can be a flooded evaporator, a direct expansionevaporator, a spray tree evaporator, a falling film evaporator or thelike. The evaporator 20 uses refrigerant provided to it by the expansionvalve 30 to condition water. The entering water is provided by a conduit70 from an air handler 72 and is measured by an entering watertemperature sensor 74. An electrical connection 76 provides the measuredentering water temperature (EWT) to the controller 60. After theentering water has undergone heat exchange in the evaporator 20, it isreturned to the air handler 72 by means of a conduit 78. The waterleaving the evaporator 20 is commonly known as leaving water. Theleaving water temperature (LWT) is measured by a leaving watertemperature sensor 80 and provided to the controller 60 by means of anelectrical connection 82. The conditioned leaving water is placed inheat exchange relationship in the air handler 72 with air that is thenprovided to zones or buildings for air conditioning purposes by means ofducts 84. The air handler 72, the conduits 70, 78 and the evaporator 20form a first water loop 86.

The refrigerant in the evaporator 20 has been vaporized by the heatexchange with the first water loop 86. As part of the refrigerant loop12, the vaporous refrigerant leaves the evaporator 20 and is directed tothe compressor 50 by a passage 90. In the compressor 50 the refrigerantis compressed. The compressor's capacity, typically measured as pumpingcapacity in CFM, is modulated by a flow control device such as thepiston unloader arrangement shown in U.S. Pat. No. 5,203,685 (previouslyincorporated by reference). Such an unloader arrangement is designatedby the reference numeral 100 and can be controlled by the systemcontroller 60 using an electrical connection 102 and solenoid valves(not shown) or the like as described in U.S. Pat. No. 5,203,685. Theunloader position is or can be directly controlled by a directpositioning device such as a stepper motor or is measured by a positionsensor 101 and reported to the controller 60 by an electrical connector103. Exemplary sensors 101 are shown in U.S. Pat. No. 5,257,921 to Clarket al. and U.S. Pat. No. 4,610,612 to Kocher, each of which areincorporated herein. Details of the unloader are shown in FIG. 7.

The compressed refrigerant leaves the compressor 50 and is directed by apassage 104 to the condenser 40. The refrigerant superheat is measuredas the differential between refrigerant temperature as measured by thesensor 92 and refrigerant temperature as measured by a sensor 106 (othermeasures of superheat are defined subsequently). Those signals arerespectively provided to the controller 60 by electrical connections 94and 108.

In the condenser 40, a cooling medium such as a second water loop 110condenses the compressed vaporous refrigerant to a liquid. The condensedliquid refrigerant is then returned to the evaporator 20 by means of apassage 120, the expansion valve 30 and a passage 122. The expansionvalve 30 is described in the patents previously incorporated herein.Effectively, the refrigerant loop 12 comprises the evaporator 20, thepassage 90, the compressor 50, the passage 104, the condenser 40, thepassage 120, the expansion valve 30 and the passage 122.

Conventionally, the measured superheat is used to modulate the amount ofrefrigerant passing through the expansion valve 30. In the presentinvention, the controller 60 provides such control of the expansionvalve 30 by means of an electrical connection 124.

The invention involves controlling the position of the expansion valve30 during the normal capacity control of the compressor 50.Conventionally, a closed loop, feedback control is operated whereby theexpansion valve 30 does not adjust to changes in compressor capacityuntil the superheat as measured by the sensor 92, 106 is affected. Thisis a feedback control since it typically occurs some minutes after anactual change in compressor capacity.

The present invention proposes to use a feed forward signal to positionthe expansion valve 30 at the time of compressor capacity changes, inanticipation of superheat changes a feed forward signal controlsundesirable effects of measurable system disturbances by appropriatelycompensating for these system disturbances before they materialize. In afirst preferred embodiment, the present invention uses the evaporatorwater temperature differential (Delta T) between the entering watertemperature EWT as measured by the sensor 74 and the leaving watertemperature LWT as measured by the sensor 80 as a feed forward signal toreposition the expansion valve 30 as the position of the unloader 100changes. This indirect measurement of compressor capacity as provided bythe evaporator water temperature differential can be used to repositionthe expansion valve 30 based on the change in load evidenced by thewater temperature differential. Secondary variations in systemdisturbance such as changes in saturated evaporator temperature orpressure, or saturated condensing temperature or pressure, will occurwhen the primary system disturbance occurs. In this case the primarysystem disturbance is a change in compressor capacity. The effects ofthese secondary variations can be used in the calculation of thefeedforward signal to the expansion valve 30.

Effectively, as long as the evaporator water temperature differential isfairly constant, the expansion valve 30 is controlled conventionally inresponse to discharge superheat as measured by the sensor 92, 106.However, should the evaporator water temperature differential vary, theexpansion valve 30 is repositioned based upon the anticipated directionand amount of change of the evaporator water temperature differential.This provides a feed forward control signal to the expansion valve 30simultaneously with the change in compressor capacity. As is discussedsubsequently in connection with FIG. 5, the conventional control of theexpansion valve 30 occurs concurrently with the feed forward control.

FIG. 2 shows a particular arrangement of the condenser 40, the expansionvalve 30 and the evaporator 20. This is a second preferred embodiment ofthe present invention which is preferred when it is desirable tominimize the amount of refrigerant charge in the refrigerant loop 12. Inthis second preferred embodiment similar reference numbers are usedwhere possible.

The second preferred embodiment of FIG. 2 includes a receiver orsubcooler 130 located in the passage 120 and intended for receiving andaccumulating, or subcooling, liquid refrigerant from the condenser 40.The expansion valve 30 controls the flow of refrigerant from thecondenser 40 and liquid receiver 130 to the evaporator 20.

Rather than the flooded evaporator 20 used in the first preferredembodiment, the evaporator 20 of FIG. 2 is a sprayed bundle evaporatorwherein liquid refrigerant accumulates in a liquid/vapor separator 132and where the liquid refrigerant is manifolded through tubing 134 to aseries of spray tubes 136. The spray tubes 136 spray refrigerant overthe tubing 138 carrying the water in the first water loop 86. Thesprayed refrigerant contacts the water tubes 138, absorbs heat therefromand exits the evaporator via passage 90 to the compressor 50. Liquidrefrigerant accumulates in a lower area 140 of the evaporator 20 wherethe depth height H1 of the refrigerant is measured conventionally, suchas by a sensor 144 connected to the controller 60 by an electricalconnection 146. If desired, the height H2 of the liquid in theliquid/vapor separator 132 can be similarly measured, as can the heightH3 of the liquid refrigerant in the condenser 40.

The second preferred embodiment of FIG. 2 modulates the expansion valve30 in response to the unloader position 100 on the theory, supported bylaboratory data, that the expansion valve's position tracks theunloader's position. Thus this embodiment contemplates feed forwardmodulation of the expansion valve 30 in response to changes in theposition 100 and feedback modulation of the position of the expansionvalve 30 in response to one of the liquid levels H1, H2 or H3(preferably H1) instead of modulating the expansion valve 30 in responseto measured superheat as would be done in a previous system.

More specifically, feed forward modulation of the expansion valve 30 isaccomplished in response to changes in the position of the unloader 100as commanded by the controller 60, and feedback modulation of theexpansion valve 30 is accomplished in response to the height H1 of theliquid refrigerant in the bottom 140 of the evaporator 20 as reported tothe controller 60 by the sensor 144. In feedback modulation, theexpansion valve 30 is incrementally opened in response to a decrease inthe height H1 of the liquid refrigerant in the bottom 140 of theevaporator 20, and is incrementally closed in response to an increase inthe height H1 of the liquid refrigerant accumulated in the bottom 140 ofthe evaporator 20.

In alternative embodiments of the second preferred embodiment shown inFIG. 2, it is contemplated that the position of the expansion valvecould be modulated in response to the height H2 of the liquidrefrigerant in the liquid/vapor separator 132 or in response to theheight H3 of the refrigerant in the condenser 40.

FIG. 3 is a block diagram showing an overview of the control of thepresent invention operating with conventional control strategy in thecontroller 60. The block diagram 200 is initiated on a regular basis oralternatively upon the occurrence of any one of a number ofpredetermined events. Such predetermined events include any change inthe position of the unloader 100, any variation in entering watertemperature or leaving water temperature, or any variation in one of theliquid refrigerant heights H1, H2 or H3.

At step 201 a determination is made as to whether an system disturbancehas occurred or is occurring. A system disturbance is a change in avariable, other than the variable used for feedback expansion valvecontrol, where that change will alter the operating conditions of thesystem. Such system disturbances be indicated by a change in compressorcapacity: as measured by the movement of the unloader 100, or asmeasured by the difference between entering water temperature andleaving water temperature. Other system disturbances include changes incondenser cooling capacity caused by fan staging, changes in setpoint,changes in cooling tower capacity, building load variations or changesin motor current. U.S. Pat. No. 5,058,031 to Swanson et al. is exemplaryof measuring motor current. This patent is assigned to the assignee ofthe present invention and is hereby incorporated by reference.

System disturbances are directed to a feed forward control 206. The feedforward control 206 is designed to cancel the effect of a systemdisturbance. The expansion valve 30 is positioned by feed forwardcontrol based on prior knowledge of the causal effect of the disturbanceon the system. In the first and second embodiment of this invention, thedisturbance causes a change in mass flow through the compressor 50. Tocancel the effect on the evaporator 20, the feed forward control 206positions the expansion valve 30 to maintain mass balance in theevaporator 20.

If there is no system disturbance, conventional feedback expansion valvecontrol 205 continues at step 202 by comparing a setpoint 203 with aconventional feedback signal 204 such as measured superheat. The controlerror is determined at block 202 and processed by the feedback controlalgorithm 205 to control the expansion valve 30. Feedback expansionvalve control 205 typically modulates the position of the expansionvalve 30 based on measured superheat but can alternatively modulatebased upon liquid level as described herein.

If a system disturbance was determined at step 201, the feed forwardcontrol 206 of the present invention is implemented. Instead ofminimizing an error signal with regard to measured superheat or liquidlevel, the feed forward control 206 will attempt to minimize an errorsignal reflective of the system disturbance. For example, if thedifference between entering water temperature and leaving watertemperature varies from a predetermined constant by an amount sufficientto be considered an error, the expansion valve 30 will immediately beadjusted to reduce that error. In a second example, any change inunloader position 100 will result in an immediate and correspondingchange in expansion valve position.

Feedback control 205 runs concurrently with feed forward control 206.Feedback control 205 reacts to disturbances in a predetermined variablesuch as superheat, as well as to setpoint changes 203 and modelingerrors in feed forward control 206. If the difference between thefeedback signal 204 and the setpoint 203 is an amount sufficient to beconsidered an error, feedback control 205 will generate an appropriatecommand signal to the expansion valve 30 to minimize the error. The feedforward signal 206 and the feedback signal 205 are summed at 207 andused to control the system 10.

FIG. 4 is a table showing the different control strategies for closedloop control as opposed to open loop control as applied to the first andsecond preferred embodiments an open loop control, unlike a closed loopcontrol, is a feed through path having no feedback around it andtherefore, is not self regulating. Feed forward controls are a type ofopen loop control.

In the first preferred embodiment, closed loop control of the expansionvalve is a feedback control based upon discharge superheat. In the eventof a system disturbance in the first preferred embodiment, a feedforward control strategy based on Delta T, the difference betweenentering water temperature and leaving water temperature, isimplemented. Both control strategies operate concurrently, however thefeed forward control strategy acts as an open loop control while thefeedback control acts as a closed loop control.

FIG. 5 is a graph demonstrating the two control strategies of the firstpreferred embodiment. Delta T, the difference between entering watertemperature and leaving water temperature, is shown as a percentage ofcapacity on the ordinate 220, while expansion valve position is shown onthe abscissa 222 as varying between minimum and maximum positions.

    capacity=MC.sub.p (Delta T)

where the M is the mass flow rate through the evaporator loop and whereC_(p) is the specific heat of water. C_(p) is a constant for a givenchiller, and M is typically assumed to be 2.4 gallons per minute per tonof mass flow. A Delta T of 10° F. is conventionally established as fullcapacity (i.e. 100%). This approach allows the control algorithm to begenerally applicable to any chiller once the appropriate C_(p) and M aredetermined. Thus, effectively, the percentage capacity shown on theordinate of FIG. 5 is determined by

    capacity=(Delta T)/(maximum Delta T)×100%.

The curve 224 shows the feed forward control where any changes in DeltaT result in an instantaneous and corresponding change in expansion valveposition along the curve 224. The sign wave 226 superimposed on thecurve 224 illustrates how the discharge superheat control continues tomodulate expansion valve position about the curve 224 even as the feedforward control based on Delta T operates. One of the advantages of thefirst preferred embodiment is in reducing wide swings in expansion valveposition which occur in prior art controls. Such wide swings areillustrated by line 228 as causing the expansion valve 30 to vary acrossits maximum and minimum positions in response to system disturbances.Such wildly varying swings are eliminated by the present invention.

Referring again to FIG. 4, the second preferred embodiment has a liquidlevel control based upon a liquid level in the evaporator as determinedby the measurement H1 or alternatively by measurement of H2 or H3. Whenthe second preferred embodiment detects a system disturbance, such as byan unloader position change, an immediate and corresponding change ismade in the position of the expansion valve 30.

This is illustrated in FIG. 6 where unloader position is shown on theordinate 240 and expansion valve (EXV) position is shown on the abscissa242. The feed forward control in response to system disturbance is shownby the curve 244. Any change in capacity made by the unloader 100, suchas an increase in capacity from point 246 to point 248, is echoedimmediately by a respective change in the expansion valve position frompoint 250 to point 252. While the broad changes in expansion valveposition are accomplished along the curve 244, the liquid level controlstrategy continues to modulate the position of the expansion valve 30 asis illustrated by the sign wave 254 superimposed upon the curve 244.

A third alternative embodiment of this anticipatory feed forward controlinvolves the comparison of the entering water temperature (EWT) asmeasured by the sensor 74 directly to the chilled water setpoint. Anywater temperature changes detected will reflect load changes in theevaporator water loop 86. The expansion valve 30 is then controlled inresponse to that detected change in load to anticipate the changes insystem capacity. Such system disturbance can be indicated by a change incompressor capacity as measured by the difference between entering watertemperature and a setpoint. In the third embodiment, the systemdisturbance is a required change in system cooling capacity below theminimum capacity obtained with the unloader 100 fully unloaded. Tocancel out the effect of the required load change, the feed forwardcontrol 206 positions the expansion valve 30 to allow refrigerant vaporto bleed out of the condenser 40 inversely proportional to the requiredload. If the entering water temperature differs from a setpoint by morethan a predetermined amount so as to be considered an error, theexpansion valve 30 will immediately be adjusted to reduce the error fromthat setpoint. Details of modulating an expansion valve while anunloader is fully unloaded are described in applicants patentapplication U.S. Ser. No. 08/234,091 filed on Apr. 28, 1994 for"Evaporator Water Temperature Control for a Chiller System" by Lee L.Sibik, Daniel C. Leaver and Paul R. Glamm now U.S. Pat. No. 5,419,146,issued on May 30, 1995. This application is assigned to the assignee ofthe present invention and is incorporated herein by reference.

FIG. 7 is a partial cross-sectional side view of a screw compressorillustrating piston unloader apparatus associated with the male rotor ofa screw compressor with the unloader piston in the full unload position.Compressor 50 is comprised of a rotor housing 312 and bearing housing314. A motor 316, male rotor 318 and female rotor (not shown) aredisposed in the rotor housing. Shaft 322 extends from the male rotor andmotor rotor 324 is mounted thereon.

Suction gas enters rotor housing 312 through the suction end 326 of thecompressor and passes through a suction strainer (not shown) prior topassing through and around motor 316 in a manner which cools the motor.In this regard, suction gas passing through and around motor 316 passesout of motor-rotor housing gap 328, rotor-stator gap 330 and intosuction area 332 within the rotor housing. The gas next passes fromsuction area 332, through suction port 334 and into the working chamber336 where it is enveloped in a chevron shaped compression pocket definedby the wall of the working chamber and the intermeshed lobes of malerotor 318 and the female rotor.

As the male and female rotors rotate, the pocket in which the suctiongas is initially enveloped is closed off from suction port 334 and iscircumferentially displaced toward high pressure end wall 338 of thecompressors working chamber. As such displacement occurs, the volume ofthe pocket is reduced and the gas contained therein is compressed untilsuch time as the pocket opens to discharge port 340.

Rotor housing 312 defines a cylindrical bore 350 which is in flowcommunication with suction port 334 or some other area of the compressoror system in which the compressor is employed which is at suctionpressure. Rotor housing 312 also defines a series of ports 352 whichcommunicate between bore 350 and working chamber 336. Disposed in bore350 is an unloader piston 354 which includes a control portion 356disposed in a chamber 358 defined by the bearing housing. Unloaderpiston 354 is axially positionable within bore 350 so as to provide forthe selective occlusion of ports 352.

From the foregoing, it is apparent that the present invention providesfeed forward controls for water chiller systems. It should be recognizedthat the invention applies to other refrigeration, HVAC, and chillersystems and that modifications are also contemplated to fall within thespirit and scope of the claims. Such modifications include thereplacement of the screw compressor described herein with a variablecapacity centrifugal compressor, a variable speed scroll compressor, avariable speed reciprocating compressor or the like. For example, theapplication of the present invention to a centrifugal chiller could useeither entering water temperature or Delta T as a closed loop, feedbackcontrol criteria and could use either compressor speed or inlet guidevane position as an open loop, feed forward control criteria. Othermodifications including the replacement of the flooded type evaporatordescribed herein with other conventional evaporators including directexpansion evaporators. Additionally, other feed forward signals arecontemplated such as the staging of fans in an air cooled condenser.Such staging is described in U.S. Pat. No. 5,138,844 to Clanin et al.That patent being commonly assigned with the present invention andincorporated by reference. It should also be recognized that the feedforward control of the expansion valve described herein is generallyexpected to operate concurrently with the conventional PID expansionvalve controls presently used. Thus, for example, the expansion valvewill modulate the flow of refrigerant based on measured superheat evenwhile the movement of the unloader generates a feed forward signalanticipating a significant change in system capacity.

Finally, although superheat has been described as the differentialbetween refrigerant temperature as measured by the sensor 92 and therefrigerant temperature as measured by the sensor 106, a person of skillin the art will recognize that there are alternative definitions ofsuperheat in use today for controlling expansion valves 30. Withreference to FIG. 1, superheat can be measured across the evaporator 20as demonstrated by sensor points 280 and 282. For the sake ofsimplicity, the electrical connections of these sensors to thecontroller 60 is omitted, but an accurate measure of superheatrepresented by the differential between sensor points 280 and 282 is inuse today as a criteria to control an expansion valve 30. A furtheralternative definition of superheat is the use of the differentialbetween the sensor 92 and the sensor point 280 to control an expansionvalve 30. Other definitions of superheat are conventionally known in theindustry and all such definitions are included in the definition ofsuperheat as used herein.

What is claims as Letters Patent of the United States are:
 1. A methodof controlling an expansion valve in a refrigeration system, the methodcomprising the steps of:controlling the modulation of the expansionvalve based upon a first feedback control criteria; and also controllingthe modulation of the expansion valve based upon a second feed forwardcontrol criteria; wherein the feedback modulation of the expansion valveis based on conventional PID control and wherein the feed forwardmodulation of the expansion valve is based on feed forward controlstrategies; and wherein the system includes an evaporator providingchilled water where the chilled water has an entering water temperature,a leaving water temperature, and a difference Delta T in temperaturebetween the entering water temperature and the leaving watertemperature, and wherein the second criteria is a function of Delta T.2. The method of claim 1 wherein the first criteria is a function ofdischarge superheat.
 3. The method of claim 1 including the further stepof determining the existence of a system disturbance.
 4. The method ofclaim 3 including the further step of initiating concurrent control ofthe expansion valve based upon the second criteria whenever a systemdisturbance is determined.
 5. A method of controlling an expansion valvein a refrigeration system, the method comprising the stepsof:controlling the modulation of the expansion valve based upon a firstfeedback control criteria; and also controlling the modulation of theexpansion valve based upon a second feed forward control criteria;wherein the feedback modulation of the expansion valve is based onconventional PID control and wherein the feed forward modulation of theexpansion valve is based on feed forward control strategies; and whereinthe system includes an evaporator providing chilled water, the chilledwater having an entering water temperature and a leaving watertemperature, and wherein the second criteria is a function of thedifference between the entering water temperature and a setpoint.
 6. Amethod of controlling an expansion valve comprising the stepsof:constantly controlling an expansion valve based upon a first measuredcriteria; determining a disturbance in a second measured criteria in thesystem being controlled by the expansion valve; providing a feed forwardcontrol signal to the expansion valve based upon the second criteria;and controlling the expansion valve based upon the second criteria;wherein the feedforward control signal is modified in response tosecondary variations in system disturbance such as changes in saturatedevaporator temperature, saturated evaporator pressure, saturatedcondenser temperature or saturated condenser pressure.
 7. A method ofcontrolling an expansion valve comprising the steps of:constantlycontrolling an expansion valve based upon a first measured criteria;determining a disturbance in a second measured criteria in the systembeing controlled by the expansion valve; providing a feed forwardcontrol signal to the expansion valve based upon the second criteria;and controlling the expansion valve based upon the second criteria; andwherein the first criteria is either measured superheat or a measuredliquid level and whether the second criteria is a measure of thetemperature of a fluid being cooled by an evaporator, or a measure ofthe cooling capacity of the system.
 8. A method of controlling anexpansion valve in a refrigeration or HVAC system comprising the stepsof:providing closed loop control of an expansion valve based upon afirst criteria; and providing open loop control of the expansion valvebased upon at least a second criteria differing from said firstcriteria; wherein the first criteria is either measured superheat or ameasured liquid level and wherein the second criteria is a measure ofthe temperature of a fluid being cooled by an evaporator, or a measureof the cooling capacity of the system.
 9. The method of claim 8 whereinthe open and closed loop controls are concurrent.
 10. A method ofcontrolling the flow of refrigerant in a refrigeration system, thesystem including an expansion device and a compression device, themethod comprising the steps of:measuring the capacity of the compressiondevice; modulating the expansion device in response to the measuredcompressor capacity; determining a measure of superheat in the system;and modulating the expansion valve based upon measured superheat until achange in measured compressor capacity exceeds predefined parameters;wherein the measured compressor capacity is determined by making ameasurement; wherein the refrigeration system includes an evaporator forcooling water, and an entering water temperature sensor for measuringthe temperature of water entering the evaporator to be cooled, andwherein the measurement is indicative of the entering water temperature.11. The method of claim 10 wherein the compression device includes aslide valve and the measurement is indicative of the slide valve'sposition.
 12. The method of claim 11 wherein the measurement isindicative of the slide valve's commanded position.
 13. The method ofclaim 10 wherein the evaporator includes a leaving water temperaturesensor measuring the temperature of water cooled by the evaporator, andwherein the measurement is indicative of the difference between theentering water temperature and the leaving water temperature.
 14. Amethod of controlling the capacity of a chiller system, the chillersystem including a circularly linked compressor, condenser, expansiondevice, and an evaporator where the evaporator receives entering waterhaving an entering water temperature and provides leaving water having aleaving water temperature and where the compressor compressesrefrigerant having a measured superheat, the method comprising the stepsof:measuring a water temperature and the refrigerant superheat;determining a water temperature error as a function of the differencebetween the water temperature and a setpoint; providing a first mode ofcapacity control wherein the expansion valve is modulated in response tothe measured superheat; and providing a second mode of capacity controlwhere the expansion valve is modulated in response to the watertemperature error.
 15. The method of claim 14 including the further stepof determining the water temperature error as a function of thedifference between the entering water temperature and the leaving watertemperature.
 16. The method of claim 14 including the further step ofdetermining the water temperature error as a function of the differencebetween the leaving water temperature and a setpoint.
 17. The method ofclaim 14 including the further step of determining the water temperatureerror as a function of the difference between the entering watertemperature and a setpoint.
 18. A method of repositioning an expansionvalve in a chiller system in response to changes in system capacity, thechiller system having an evaporator for conditioning entering water andproviding leaving water, the method comprising the steps of:monitoringthe entering water temperature of the evaporator; monitoring the leavingwater temperature of the evaporator; determining a water temperaturedifference between the entering water temperature and the leaving watertemperature; determining a temperature differential setpoint; comparingthe water temperature difference to the differential temperaturesetpoint to establish an error; and repositioning the expansion valve tominimize the error.
 19. A water chiller system comprising an evaporatorfor chilling a fluid;a compressor, receiving heated refrigerant from theevaporator, for changing the condensation temperature of the refrigerantby compression; a condenser, receiving compressed refrigerant from thecompressor, for condensing the compressed refrigerant; an expansiondevice, receiving condensed refrigerant from a condenser, forcontrollably returning the condensed refrigerant to the evaporator; acontroller, operatively connected to the expansion device, forcontrolling the operation of the expansion device; and feed forwardcontrol, coupled to the controller, for anticipating changes in the loadof the evaporator whereby the feed forward control includes a firstsensor sensing a first value and controlling the operation of theexpansion valve in response to the sensed first value, the first sensorbeing operably connected to the controller and measuring the temperatureof the first fluid as it enters the evaporator, and wherein the enteringfluid temperature is said first value.
 20. The chiller system of claim19 further including the first temperature sensor operably connected tothe controller and measuring the temperature of the fluid entering theevaporator, a second temperature sensor operably connected to thecontroller and measuring the temperature of the fluid leaving theevaporator, and a comparator operatively associated with the controllerand determining a differential between the temperatures measured by thefirst and second sensors whereby that differential is the first value.21. The chiller system of claim 19 wherein the compressor includes aslide valve and a device operatively associated with the controller anddetermining the position of the slide valve whereby the position of theslide valve is said first value.
 22. The chiller system of claim 21further including a second sensor in association with the evaporator andoperatively linked with the controller and determining the height ofliquid refrigerant in the bottom of the evaporator, whereby the measuredheight of the liquid in the bottom of the evaporator is used as a secondvalue for controlling the position of the expansion device.
 23. Thechiller system of claim 19 wherein the evaporator includes a liquidvapor separator and wherein the first sensor determines the amount ofliquid refrigerant accumulating in the liquid vapor separator andprovides a signal representative of that measured amount to thecontroller whereby the first value is the measured amount of liquidrefrigerant in the liquid vapor separator.
 24. The chiller system ofclaim 19 wherein the first sensor measures the accumulation of liquidrefrigerant in the bottom of the condenser and forwards a signalrepresentative of that accumulation to the controller whereby themeasured accumulation of liquid refrigerant in the condenser is thefirst value.